Three Letter Non-Plurals

Note: These lists exclude words containing heavy letters, as well as words considered to be very common

Hooks Which Cannot be Pluralised with (E)S

ACH	Scots. interj. expressing regret or impatience (also OCH)
AHA	interj. expressing exultation or pleasure
ARY	dial. for any
ATT	a Laotian monetary unit (pl. ATT)
AWA	Scots. for away
BAH	interj. expressing disgust or contempt
BON	good (Fr.)
CHE	dial. form of I, used by Shakespeare
CIS	having certain atoms on the same side of the molecule
CUM	with the addition of
DEF*	excellent, brilliant
DEI	the plural of DEUS (Latin for god)
EEK	interj. expressing fright
EEN	the plural of EE (Scots. for eye)
ERF	a South African garden plot (pl. ERVEN)
FAB*	marvellous
FAP	fuddled or drunk  
FER	for
GAN	a past tense of gin, to begin
GIF	arch. form of if
GOR	interj. used as a mild oath
HAN	Spens. plural of have
HAO	a monetary unit of Vietnam (pl. HAO)
HIC	interj. representing a hiccup
HOC	Latin word for this
HOI	interj. used to attract attention
HOO	Shak. interj. expressing boisterous emotion
HMM	same as HM (interj. expressing thoughtful consideration)
HUH	interj. expressing non-understanding
IFF	'if and only' if' as used in logic
KAM	awry, twisted, distorted  
KOI	a large Japanese carp (pl. KOI)
KYE	same as KY (cows, cattle)
LOR	colloq. for lord, interj. expressing surprise
MED	medical
MIM*	prim and proper  
MOI	French form of me, facetiously used in English
NAE	Scots. form of no, certainly not
NAH	colloq. for no
NAW	colloq. for no
NEE	(of a woman) born
NIS	a brownie or goblin in Scandinavian folklore (pl. NISSES)
NOH	a traditional style of Japanese drama
NON	Latin word for not
NOO	now
OCH	same as ACH, above
OFT*	poetic for often
OOR	Scots. for our
ORA	the plural of OS (a bone)
PHT	interj. of mild anger
PIU	more (Italian)
POH	interj. expressing contempt
PRE	before
PST	interj. used to attract attention (also PSST)
PUH	Shak. for pooh, interj. expressing disgust
SAE	so
SAM	together  
SED	Milt. spelling of said
SHA	same as SH, SHH (interj. requesting silence)
SHH	see SHA, above
SMA	Scots. word for small
SUR	on, above (Fr.)
THO	Spens. for those
UMM	same as UM (interj. expressing doubt or hesitation)
UPO	upon
WHA	Scots. form of who
YAR	nimble (also YARE)
YEH	colloq. form of yes (also YEAH)
YGO	Spens. past tense of go
YOM	a Jewish day (pl. YOMIM)
YON	yonder
YUM	interj. to express satisfaction

Other Threes Which Cannot be Pluralised with (E)S

AFF	off
BRR	interj. to indicate cold (also BRRR)
DUI	a plural of duo
FIE	an old interj. expressing disapproval
FRO	obs. form of from
GEY*	considerable
ICK	interj. used to express disgust
LEU	a unit of Romanian currency (pl. LEU)
LEV	a unit of Bulgarian currency (also LEW; pl. LEVA)
LEW	same as LEV, above
NTH	implying a large number
OVA	the plural of ovum, an egg
SAU	same as XU (a Vietnamese coin; pl. SAU)
SUI	Latin word meaning of himself, itself
SYN	since
TWP	Welsh word for dim-witted or stupid
UDS	old interj. meaning "God Save"